Multi-zonal monofocal intraocular lens for correcting optical aberrations

ABSTRACT

A multi-zonal monofocal opthalmic lens comprises an inner zone, an intermediate zone, and an outer zone. The inner zone has a first optical power. The intermediate zone surrounds the inner zone and has a second optical power that is different from the first power by a magnitude that is less than at least about 0.75 Diopter. The outer zone surrounds the intermediate zone and has a third optical power different from the second optical power. In certain embodiments, the third optical power is equal to the first optical power.

RELATED APPLICATION

The present application claims priority under 35 U.S.C §119(e) toprovisional application No. 60/424,851, filed on Nov. 8, 2002 under thesame title. Full Paris Convention priority is hereby expressly reserved.

FIELD OF THE INVENTION

This invention relates to intraocular lenses (IOLs) and, moreparticularly, to multi-zonal monofocal IOLs that correct opticalaberrations for a variety of human eyes with different corneas under awide range of lighting conditions and that are effective even whendecentered or tilted.

BACKGROUND OF THE INVENTION

In the perfect eye, an incoming beam of light is focused through thecornea and through the crystalline lens in a way that causes all of thelight from a point source to converge at the same spot on the retina ofthe eye, ideally on the fovea area of the retina. This convergenceoccurs because all of the optical path lengths, for all light in thebeam, are equal to each other. Stated differently, in the perfect eyethe time for all light to transit through the eye will be the sameregardless of the particular path that is taken by the light.

Not all eyes, however, are perfect. The consequences of this are thatlight path lengths through the eye become distorted and are not allequal to each other. Thus, light from a point source that transits animperfect eye will not necessarily come to the same spot on the retinaand be focused.

As light enters and passes through an eye it is refracted at theanterior surface of the cornea, at the posterior surface of the cornea,and at the anterior and posterior surfaces of the crystalline lens,finally reaching the retina. Any deviations that result in unequalchanges in these optical path lengths are indicative of imperfections inthe eye that may need to be corrected. For example, many people arenear-sighted because the axial length of their eyes are “too long”(myopia). As a result, the sharp image of an object is generated not onthe retina, but in front of or before the retina. Hyperopia is acondition where the error of refraction causes rays of light to bebrought to a focus behind the retina. This happens because the axiallength is “too short”. This condition is commonly referred to asfarsightedness. Another refractive malady is astigmatism resulting froma refractive surface with unequal curvatures in two meridians. Thedifferent curvatures cause different refractive powers, spreading lightin front and in back of the retina.

Other “higher order” maladies of interest for vision correction includecoma and spherical aberration. Coma exists when an asymmetry in theoptical system causes unequal optical path lengths in a preferreddirection. For example, the image of an off-axis point object takes on acomet-like shape. For symmetrical systems, spherical aberration existswhen rays at different radial heights from the optical axis focus atdifferent axial locations near the retina. Whereas coma exists only inasymmetric systems, spherical aberration can exist in both symmetric andasymmetric systems. Other, even higher order, aberrations exist.However, studies have show that spherical aberration is one of thestrongest higher order aberrations in the human visual system. Thus theretinal image may be improved if the spherical aberration is correctedaccording to known techniques.

Studies have also shown that there is a balance between the positivespherical aberration of the cornea and the negative spherical aberrationof the crystalline lens in younger eyes. As one grows older, thespherical aberration of the crystalline lens becomes more positive,increasing the overall spherical aberration and reducing the imagequality at the retina.

An intraocular lens (IOL) is commonly used to replace the natural lensof a human eye when warranted by medical conditions such as cataracts.In cataract surgery, the surgeon removes the natural crystalline lensfrom the capsular bag or posterior capsule and replaces it with an IOL.IOLs may also be implanted in an eye (e.g., in the anterior chamber)with no cataract to supplement the refractive power of the naturalcrystalline lens, correcting large refractive errors.

The majority of ophthalmic lenses including IOLs are monofocal, or fixedfocal length, lenses that primarily correct refractive error. Mostmonofocal IOLs are designed with spherical anterior and posteriorsurfaces. The spherical surfaces of the typically positive power IOLscause positive spherical aberration, inter alia. Thus, replacement ofthe crystalline lens with a typical monofocal IOL leaves the eye withpositive spherical aberration. In real eyes with complex cornealaberrations, the eye following cataract surgery is left a with finitenumber of complex lower and higher order aberrations, limiting the imagequality on the retina.

Some examples of attempts to measure higher order aberrations of the eyeas an optical system in order to design an optical lens include U.S.Pat. No. 5,062,702 to Bille, et al., U.S. Pat. No. 5,050,981 to Roffman,U.S. Pat. No. 5,777,719 to Williams, et al., and U.S. Pat. No. 6,224,211to Gordon.

A typical approach for improving the vision of a patient has been tofirst obtain measurements of the eye that relate to the topography ofthe anterior surface of the cornea. Specifically, the topographymeasurements yield a mathematical description of the anterior surface ofthe cornea. This corneal surface is placed in a theoretical model of thepatient's eye with an IOL replacing the crystalline lens. Ray-tracingtechniques are employed to find the IOL design which corrects for thespherical aberration of the cornea. Ideally, if implanted with thiscustom IOL, the patient's vision will improve.

Recently, Pharmacia Corp. (Groningen, Netherlands) introduced aposterior capsule intraocular lens having the trade name TECNIS (Z9000)brand of Silicone IOL. The TECNIS lens has a prolate anterior surface,which is intended to reduce spherical aberrations of the cornea. Thislens may be designed using methods described in U.S. Pat. No. 6,609,793and PCT publication WO 01/89424, both to Norrby, et al. The methods inthese publications involve characterizing aberrant corneal surfaces aslinear combinations of Zernike polynomials, and then modeling orselecting an intraocular lens which, in combination with acharacteristic corneal surface, reduces the optical aberrations ocularsystem. The lenses resulting from these methods may be continuousaspherical surfaces across the entire optical zone and may be used toreduce spherical aberrations of the eye by introducing negativespherical aberration to counter the typically positive sphericalaberration of the cornea. In these lenses, there may be a single basecurve on which the aspheric surface is superimposed. As reported by J.T. Holliday, et al., “A New Intraocular Lens Designed to ReduceSpherical Aberration of Pseudophakic Eyes,” Journal of RefractiveSurgery 2002, 18:683-691, the Technics IOL has been found to be toimprove visual contrast sensitivity at a frequency up to 18cycles/degree.

The TECNIS brand of lens generally requires precise positioning in thecapsular bag to provide improved optical quality over a spherical IOL(c.f., “Prospective Randomized Trial of an Anterior Surface ModifiedProlate Intraocular Lens,” Journal of Refractive sugery, Vo. 18, Nov/Dec2002). Slight errors in decentration (radial translation) or tilt (axialrotation) greatly reduces the effectiveness of the lens, especially inlow-light conditions, thus making the task of the surgeon moredifficult. Furthermore, shrinkage of the capsular bag or otherpost-implantation anatomical changes can affect the alignment or tilt ofthe lens along the eye's optical axis. It is believed that the “typical”magnitude of decentration resulting from the implantation of anintraocular lens in an average case, and factoring in post-implantationmovement, is less than about 1.0 mm, and usually less than about 0.5 mm.Most doctors agree that decentration of an IOL greater than about 0.15to approximately 0.4 mm is clinically relevant (i.e., noticeably affectsthe performance of the optical system, according to those skilled in theart). Similarly, the “typical” magnitude of tilt resulting from theimplantation of an intraocular lens in an average case, and factoring inpost-implantation movement, is less than about 10 degrees, and usuallyless than about 5 degrees. Therefore, in practice, the benefits of theTECNIS brand of lens may be offset by its apparent drawbacks in the realworld.

In view of the above, there remains a need for an intraocular lens thatcorrects for spherical aberrations in a variety of lighting conditionsand is less sensitive to non-optimal states such as decentration andtilt of the IOL.

SUMMARY OF THE INVENTION

The present invention provides a multi-zonal monofocal ophthalmic lensthat is less sensitive to its disposition in the eye by reducingaberrations, including the spherical aberration, over a range ofdecentration. The monofocal ophthalmic lenses of the present inventionmay also be configured to perform well across eyes with differentcorneal aberrations (e.g., different asphericities).

In one aspect of the invention, a multi-zonal monofocal opthalmic lenscomprises an inner zone, an intermediate zone, and an outer zone. Theinner zone has a first optical power. The intermediate zone surroundsthe inner zone and has a second optical power that is different from thefirst power by a magnitude that is less than at least about 0.75Diopter. The outer zone surrounds the intermediate zone and has a thirdoptical power different from the second optical power. In certainembodiments, the third optical power is equal to the first opticalpower. The ophthalmic lens may comprise between 3 and 7 total zones, butfavorably comprises between 3 and 5 total zones. However, ophthalmiclenses with more than seven total zones are consistent with embodimentsof the invention.

In another aspect of the invention, a multi-zonal monofocal intraocularlens has an optic with a plurality of concentric optical zones centeredon the optical axis. The zones are adapted to focus incoming light raysto form the image from one object. The intraocular lens optic includesan inner zone overlapping the optical axis of the lens that provides animage when the intraocular lens is centered on the optical axis of thehuman eye. A first surrounding zone concentric about the inner zone isadapted to compensate for optical aberrations resulting from implantedintraocular lens decentration of greater than at least about 0.1 mm.

The first surrounding zone may be configured to compensate for opticalaberrations resulting from implanted intraocular lens decentration ofgreater than at least about 0.1 mm. The first surrounding zone may alsocompensate for optical aberrations resulting from implanted intraocularlens tilt of greater than at least about 1 degree. The power of thefirst surrounding zone preferably differs from the power of the innerzone by a magnitude that is less than or equal to at least about 0.75Diopter. In an exemplary embodiment, the inner zone comprises aspherical surface and the first surrounding zone comprises an asphericalsurface.

Another aspect of the invention includes a method of designingmulti-zonal monofocal opthalmic lens. The method comprises providing anoptical model of the human eye. The method further comprises an opticalmodel of a lens comprising an inner zone, an intermediate zone, an outerzone, and zonal design parameters. The method also comprises adjustingthe zonal design parameters based on an image output parameter for oneor more non-optimal states of the lens.

The method may further include testing the intraocular lens over a widerange of clinically relevant corneal surface variations and dispositionsof optical elements in the eye's optical system using ray-trace analysistechniques. Furthermore, the method may be repeated to modify zonalparameters and achieve a better average optical performance. Examples ofconditions of asymmetry that the lens will correct include decentration,tilt, and corneal aberrations.

The invention, together with additional features and advantages thereof,may best be understood by reference to the following description takenin connection with the accompanying illustrative drawings in which likeparts bear like reference numerals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic vertical cross-section of the human eye in abright light environment and showing a pair of light rays passingthrough the optical system of the cornea and an implanted intraocularlens of the prior art to focus on the retina.

FIG. 2 is a schematic vertical cross-section of the human eye in a lowlight environment and showing a pair of light rays passing through theoptical system of the cornea and the peripheral regions of an implantedintraocular lens of the prior art to focus in front of the retina.

FIG. 3 is a schematic vertical cross-section of the human eye in abright light environment and showing a pair of light rays passingthrough the optical system of the cornea and a decentered implantedintraocular lens of the prior art to focus on the retina.

FIG. 4 is a schematic vertical cross-section of the human eye in amedium light environment and showing a pair of light rays passingthrough the optical system of the cornea and a decentered implantedintraocular lens of the prior art to focus in front of the retina.

FIGS. 5A and 5B are schematic plan and side views of a monofocalintraocular lens of the present invention illustrating concentric zonesabout an optical axis.

FIGS. 6A and 6B show simulated modulation transfer functions for anaspheric, spherical and multi-zonal monofocal IOLs at a 5 mm pupildiameter with no decentration and 0.5 mm decentration, respectively.

FIG. 7 show simulated aspheric, spherical, and multi-zonal monofocal IOLMTF curves at a 5 mm pupil diameter representing the respective averageMTFs over 100 eyes varying in corneal aberrations, IOL decentration andtilt, and small pupil size changes.

DETAILED DESCRIPTION

The present invention encompasses an intraocular lens (IOL) design thatreduces sensitivity to decentration within the eye while maintainingsuperior Module Transfer Function (MTF) performance for large pupils.The MTF is a measure of visual performance that can be plotted on anon-dimensional scale from a minimum of 0.0 to a maximum of 1.0 across arange of spatial frequencies in units of cycles per mm. The MTF is ameasure of the efficiency of “transferring” the contrast of an objectinto an image. The spatial frequency is inversely proportional to thesize of the object. Thus, small objects at the limit of visualresolution have high spatial frequencies than larger objects. The IOLdescribed herein comprises a multi-zonal monofocal lens in which theanterior lens surface, posterior lens surface, or both comprises aplurality of zones that operate together on an incident wavefront toproduce a corrected ocular image. The different zones of the IOL of thepresent invention, as described in greater detail below herein,generally have different mean spherical curvatures and/or Diopterpowers, but the Diopter power differences between zones are far lessthan the typical 2 Diopter to 4 Diopter design differences associatedwith multi-focal IOLs. In certain embodiments, the maximum Diopter powerdifference between any two zones is less than at least about 0.75 D,advantageously less than about 0.65 D.

As used herein, the term “monofocal lens” is considered to be a lens inwhich light entering the lens from a distant point source is focused tosubstantially a single point. In the case of a multi-zonal monofocallens, light from a distant point source entering the lens zonessubstantially fall within the range of the depth-of-focus of a sphericallens having an equivalent focal length.

As used herein in reference to the zones of a multi-zonal monofocallens, the terms “optical power” and “Diopter power” refer to theeffective optical or Diopter power of a zone when the lens is part of anocular lens system such as, for example, a cornea, a multi-zonalmonofocal IOL, a retina, and the material surrounding these components.This definition may include the effects of the vergence or angle oflight rays intersecting the IOL surface caused by the power of thecornea. This may include the total vergence from all optical surfaces infront of the multi-zonal monofocal IOL. In certain instances, analgorithm for calculating the Diopter power may begin with a ray-tracinga model of the human eye incorporating a multi-zonal monofocal IOL. At aparticular radial location on the IOL surface, Snell's law may beapplied to calculate the angle of the light ray following therefraction. The optical path length of the distance between a point onthe surface and the optical axis (axis of symmetry) may be used todefine the local radius of curvature of the local wavefront. Using suchan approach, the Diopter power is equal to the difference in indicies ofrefraction divided by this local radius of curvature.

IOLs of the present invention are designed to outperform certain IOLs ofthe prior art in low or moderate light situations over a larger range ofimplant positions. In practice, clinicians recognize that in the averagecase intraocular lenses implanted in the posterior capsule end updecentered from the optical axis of the host eye by between about0.15-0.4 mm. Sometimes the decentration is greater as a result of poorimplant technique or non-axisymmetric forces imparted by the host eye.Indeed, decentration of more than 0.5 mm, and sometimes up to 1.0 mm isexperienced. IOLs of the present invention are specifically designed toexhibit superior performance in comparison to the prior art IOLs whendecentered by at least about 0.15 mm and in particular in low ormoderate light conditions. In certain embodiments, IOLs of the presentinvention are designed to exhibit superior performance in comparison toprior art IOLs when decentered by greater than about 0.5 mm or greaterthan about 1.0 mm. The amount of decentering to be accommodated dependsupon design constraints such as, for example, the accuracy of thesurgical method to be used for implanting the IOL. Since the multi-zonalmonofocal IOLs provide improved performance for decentered conditions,it is anticipated that patients will generally experience greatersatisfaction with a multi-zonal monofocal IOL than with other prior artIOLs.

FIG. 1 is a schematic vertical cross-section through a human eye 20having an IOL 22 of the prior art implanted therein. The optical systemof the eye 20 includes an outer cornea 24, a pupil 26 defined by anorifice of an iris 28, the IOL 22, and a retina 30 formed on theposterior inner surface of the ocular globe 32. In the presentapplication, the terms anterior and posterior are used in theirconventional sense; anterior refers to the front side of the eye closerto the cornea, while posterior refers to the rear side closer to theretina. The eye defines a natural optical axis OA. The drawing shows theeye 20 in a bright light environment with the iris 28 constrictedresulting in a relatively small pupil 26.

The exemplary IOL 22 is adapted to be centered along the optical axis OAand within a capsular bag (not shown) just posterior to the iris 28. Forthis purpose, the IOL 22 may be provided with haptics or fixationmembers 34. An optic of the IOL 22 is defined by an anterior face 36 andposterior face 38. The optic may take a variety of configurations knownin the art, such as the convex-convex configuration illustrated in FIG.5B. It should be understood that the present invention is not limited toposterior capsule-implanted IOLs.

A pair of light rays 40 pass through cornea 24, pupil 26, the IOL 22.The rays 40 then focus on the retina 30 along the optical axis OA. Inthe bright light environment shown, the light rays 40 pass through themid-portion of the lens optic. The intraocular lenses of the prior artare relatively effective in focusing such light rays at a point on theretina 30 along the optical axis OA.

FIG. 2 shows the eye 20 having the IOL 22 therein in a low lightenvironment. In such situations, the iris 28 opens up creating arelatively large pupil 26 and permitting more light to strike the IOL22. A pair of light rays 42 passing through the peripheral regions ofthe pupil 26 may be incorrectly refracted by the peripheral regions ofthe optic of the IOL 22 in the manner shown. That is, the light rays 42focus on a spot 44 along the optical axis OA that is in front of theretina 30 by a distance 46. Such refraction is termed positive sphericalaberration because the light rays 42 focus in front of the retina 30. Anegative spherical aberration focuses light rays at the imaginary pointalong the optical axis OA behind the retina 30. Such aberrations canalso occur in an eye with the natural lens still in place. For example,the crystalline lens in the aging eye may not refract light properlyunder low light environments. The practical result of such a conditionmay be a loss in image quality.

FIG. 3 illustrates the human eye 20 in a bright light environment suchas shown in FIG. 1. The IOL 22 centered along the optical axis OA isagain shown in solid line, but is also shown in dashed line 22′representing a condition of decentration. As mentioned above,decentration involves a radial translation of the intraocular lens froma centered configuration on the natural optical axis OA. The light rays40 pass through the cornea 24 and relatively small pupil 26, and arerefracted through the central region of the decentered intraocular lensoptic 22′. That is, despite the undesirable decentration, the optic 22′performs well in bright light environments because light does not strikeand refract through its peripheral regions.

FIG. 4 illustrates the eye 20 in a medium light environment, in whichthe iris 28 is somewhat larger compared to the condition shown in FIG.3, but is not fully expanded as seen in the low light environment ofFIG. 2. Under such conditions, a centered IOL 22 would likely performadequately, but the decentered lens 22′ will not. More particularly, alight ray 48 passing close to the iris 28 will strike and be incorrectlyrefracted through a peripheral region of the decentered optic 22′ asshown. Intraocular lenses of the prior art have varying degrees ofsensitivity to decentration, and the situation shown in FIG. 4 is forillustration purposes only and does not represent any particular lens.

However, it is believed that certain lenses designed to correct forspherical aberration, such as the TECNIS brand of lens, are relativelysensitive to small magnitudes of decentration. Such lenses have acomplex refractive surface that changes relatively continuously acrosswhichever face it is formed (i.e., anterior or posterior). Thiscontinuous refractive surface provides a negative correction for thepositive spherical aberration on the cornea, but when the lens isdecentered the closely calculated balance between the two opticaldevices may be lost. Indeed, other optical aberrations such as coma andastigmatism may be created by the resulting mismatch.

FIGS. 5A and 5B schematically illustrate in plan and side views amonofocal IOL 60 of the present invention having an optic 62 and a pairof haptics or fixation members 64 a, 64 b extending outward therefrom.The optic 62 has a generally circular peripheral edge 66 and a pluralityof concentric annular refractive bands or zones formed thereon. Theperipheral edge 66 is desirably an axially oriented edge with thickness,as seen in FIG. 5B, although curved or angled edge surfaces, orcombinations thereof, are possible. The optic 62 has an anterior face 68a and an opposite posterior face 68 b separated by the peripheral edge66. It should be understood that the refractive zones can be formed oneither the anterior or posterior face, or in some cases as a combinationof both faces. A central and inner zone 70 centered on the optical axisOA extends outward to a radius of r₁, at least one intermediate zone 72surrounds the inner zone 70 and extends outward to a radius of r₂, andan outer zone 74 surrounds the intermediate zone 72 and extendstherefrom to the outer periphery 66 of the optic 62 and a radius of r₃.Desirably, r₁ is between about 1-1.5 mm, r₂ is between about 1.5-2.2 mm,and r₃ is about 3 mm. More desirably, r₁ is about 1.4 mm and r₂ is about2.0 mm. certain instances, it may be desirable that r₃ is greater than 3mm, for instance in order to preclude undesired edge effects.

The inner zone 70, intermediate zone 72, and outer zone 74 may havesurfaces that are either spherical or aspherical in shape. Theintermediate zone 72 may comprise a combination of annular zones,although a single annular zone is generally desirable. In certainembodiments, the inner zone 70 is spherical, the intermediate zone 72 isaspherical, and the outer zone 74 is also aspherical.

The power of the inner zone 70 dominates the visual performance of theeye when the pupil is small, such as in bright daylight situations. Theintermediate zone 72 is at least designed to help correct aberrations ofthe IOL when it is decentered, tilted, or otherwise in a non-optimalstate. The power of intermediate zone 72 is extremely close to that ofthe inner zone 70. The outer zone 74 may be aspherical and designed tominimize the spherical aberrations natural to spherical monofocal IOLs.

Preferably, the intermediate zone 72 has a correction power that is lessthan the correction power of the inner zone 70. When a prior art IOL isdecentered (FIG. 4), peripheral light is too strongly refracted andfocuses in front of the retina. However, the intermediate zone 72 of themulti-zonal monofocal IOL 60 is used to reduce surface power,redirecting the light ray 48 to the focal point on the retina. Theintermediate zone 72 may also provide correction in cases of tilting ofthe lens within the typical range of at least about 1 to 10 degrees,depending upon design constraints such as, for example, the accuracy ofthe surgical method to be used for implanting the IOL.

The IOL 60 is considered to be a monofocal lens because the relativerefractive powers of the zones 70, 72, and 74 are close to one anotherand within the range of the depth-of-focus of typical sphericalmonofocal IOLs. In this context, a “monofocal” lens is one in, whichdiscrete adjacent regions or zones have a maximum difference inrefractive power of less than at least about 0.75 Diopter. Therefractive power of any one zone may be interpreted as the mean powerwithin that zone. It should also be understood that discrete adjacentzones does not necessarily mean that there is a sharp physicaltransition therebetween, rather the manufacturing process may bedesigned to generally provide a smooth transition between adjacentzones.

The IOL 60 may be fabricated from materials used in the art, such assilicon, acrylic, or Polymethylmethacrylate (PMMA), or any othermaterial that is suitable for use in or on a human eye. Materials mayalso be selected so as to provide a desired optical performance. Forinstance, the refractive index is known to vary with different materialsand may, therefore, be used as a design parameter for attaining adesired optical performance or affect from the IOL 60.

The IOL 60 may also be used in conjunction with other optical devicessuch as diffractive optical elements (DOE). For example, the anteriorlens surface of the IOL 60 may comprise a multi-zonal surface and theposterior lens surface may contain a DOE such as a diffractive grating,or visa versa. Alternatively, the multi-zonal surface itself maycomprise a DOE such as a diffractive grating. The DOE may also be used,for example, to correct for chromatic aberrations or to improve theperformance of the IOL 60 when displaced from the optimal position(e.g., centered and normal to the optical axis). In certain embodiments,the DOE is disposed over only a portion of the one of the IOL surfaces.For example, the DOE may be disposed over the intermediate zone 72 andused as an additional parameter for improving the performance of the IOL60.

The IOL 60 may be designed to have a nominal optical power suited forthe particular environment in which it is to be used. It is anticipatedthat the nominal optical power of the IOL 60 will generally be within arange of about −20 Diopters to at least about +35 Diopters. Desirably,the optical power of the IOL 60 is between about 10 Diopters to at leastabout 30 Diopter. In certain applications, the optical power of the IOL60 is approximately 20 Diopters, which is a typical optical power forthe natural crystalline lens in a human eye.

Under low light environments, such as night-time, the human eye has alarger pupil (about 4.5-6 mm in diameter) and hence has a largespherical aberration (SA) that blurs the image. Clinically, thelarge-pupil eye is reported to have a lower contrast sensitivity andsometimes lower visual acuity. The TECNIS brand of lens has beenreported to perform better than spherical IOLs in low light environmentsas judged by visual contrast sensitivity and visual acuity. According tosimulations, however, this aspherical design is sensitive todecentration. A fraction of a millimeter decentration of such IOLs fromthe optical axis may dramatically break the balance of SA between IOLand cornea, and thus seriously degrade the eye's vision.

The inventors have discovered that spherical aberration can be reducedfor both on-design and off-design conditions by forming a lens surfaceto have a multi-zonal structure, with each zone having different surfaceparameters, for example, the base radius of curvature. In contrast withthe prior art single continuous aspheric surface, such as the TECNISbrand of lens described above, the surface sag of the IOL 60 (i.e.multi-zonal surface contour) may be determined using an equation thatchanges across the lens. In accordance with an exemplary embodiment ofthe present invention, the surface sag at any radius from the opticalaxis for an ith zone is given by the following equation:

${Sag} = {\frac{C_{i}*r^{2}}{1 + \sqrt{1 - {( {1 + K_{i}} )*C_{i}^{2}*r^{2}}}} + {\sum\limits_{j = 0}^{M}\;{B_{i\; j}*( {r - r_{i}} )^{2j}}} + {\sum\limits_{j = 1}^{M}\;{T_{i\; j}*( {r - r_{i - 1}} )^{2j}}}}$

where C_(i), K_(i), and r_(i) are the base radius of curvature, theasphericity constant, and the height of the ith zonal surface. Further,the Bjs and Tjs are optional boundary parameters that can be used toconnect the zonal surfaces smoothly. The variable M is an integer thatdetermines how smoothly one zone transitions to another. This work makesuse of a published finite eye model to represent the “nominal” eye forIOL design (see, Liou H. L. and Brennan N. A., “Anatomically Accurate,Finite Model Eye for Optical Modeling, J Opt Soc Am A, 1997;14:1684-1695).

For posterior chamber IOL design, the asphericity constant K₁ in theinner zone 70 (FIG. 5A) is preferably zero (i.e., the inner zone 70comprises a spherical surface). The base radius of curvature C₁ in theinner zone 70 is considered to be the base surface power of the lens.There are preferably at least three zones (i≧3) to achieve enhancedperformance for a 6 mm diameter pupil size. A preferred range of thenumber of zones is between at least about 3-7, more preferably between3-5; however, larger numbers of zones may be used of particular designconditions. The parameters in the outlying zones can be optimallydetermined such that each zonal surface refracts more of the light raysin that particular zone to the focus set by the inner zone. This processcan be achieved by the aid of a commercial optical ray tracing designsoftware, such as ZEMAX optical design program from ZEMAX DevelopmentCorporation (4901 Morena Blvd. Suite 207, San Diego, Calif. 92117-7320).

In general, the base curves in at least two zones are different(preferably the inner and intermediate zones), though all zones may havedifferent base curves. Desirably, the anterior surface has three zones,each having a different base radius of curvature. The posterior surfaceis a one zone spherical surface.

Table 1 provides an example of a multi-zonal monofocal IOL consistentwith the present invention. The values of the parameters given below arefor an IOL with an overall Diopter power of 20 having 3 zones (i=3) onthe anterior surface and one zone on the posterior (i=1).

TABLE I Surface parameters of a 20D multi-zonal structured IOL Symbol i= 1 i = 2 i = 3 Anterior surface parameter Zonal outer radial r_(i) (mm)1.414 2.000 3.000 boundary, mm Zonal curvature of C_(i) 0.086149000000.0751110000000 0.05055500000000 radius, 1/mm (1/mm) Zonal asphericityK_(i) 0.00000000000000 −1.5931120000000 8.90504900000000 M = 3 B_(i0)0.00163052185449 0.01542174622418 0.11151991935001 B_(i1)−0.0024465216312 −0.0241315485668 −0.0611825408097 B_(i2)0.00122363035200 0.08421200000000 0.00963200000000 B_(i3)−0.0002040000000 −0.1293190000000 0.00399800000000 T_(i1)0.00000000000000 .02774300000000 −0.0571790000000 T_(i2)−0.0004750000000 −0.1375720000000 0.13027200000000 T_(i3)0.00007700000000 0.23032800000000 −0.0800460000000 Posterior surfaceparameter Zonal outer radial r_(i) (mm) 3.000 boundary, mm Zonalcurvature of C_(i) 0.0636027120000 radius, 1/mm (1/mm) Zonal asphericityK_(i) 0.00000000000000 M = 0 N/A Notes: 1. IOL refractive index at 35°is 1.47; 2. IOL central thickness is 0.977 mm. 3. IOL nominal base power= 20D

FIGS. 6A and 6B illustrate the IOL performance the multi-zonal monofocallens shown in Table 1 in terms of the simulated modulation transferfunctions as compared to both a spherical lens and an aspheric lens (theTECNIS brand of lens). These simulated results are based on a 5 mm pupildiameter with no decentration (FIG. 6A) and 0.5 mm decentration (FIG.6B). FIG. 6A illustrates the performance for each type of lens when thelenses are precisely centered within the eye. In FIG. 6B, theperformance of each type of lens is illustrated when the lens isdecentered from the optical axis of the eye by 0.5 mm, a condition thatis not uncommon under realistic conditions.

In comparing FIG. 6B to FIG. 6A, it can be seen that with decentration,both the aspheric and multi-zonal monofocal designs suffer a large lossin image quality (e.g., MTF). However, the multi-zonal loss is lesscompared to the aspheric design. Observe in FIG. 6A that the asphericand multi-zonal MTFs are significantly higher compared to the standardspherical surface design. The price paid for the significant enhancementof image quality is the sensitivity to non-nominal conditions (e.g.,decentration) shown in FIG. 6B. However, some improvement in thenon-nominal condition can be achieved by this novel use of zones in thedesign of an improved monofocal IOL. The price paid for the reduction innon-nominal sensitivity is the slightly lower multi-zonal design MTFcompared to the aspheric MTF shown in FIG. 6A. Never-the-less, themulti-zonal MTF remains significantly improved compared to the sphericaldesign MTF.

FIG. 7 illustrates the results of a Monte Carlo simulation in the formof plots of the average MTF performance for spherical, aspheric, andmulti-zonal monofocal IOLs based on over 100 different eyes and undervarying conditions of corneal aberrations, IOL decentration, and IOLtilt. The simulation was conducted using a 5 mm nominal pupil diameter.The results compare the average performance of the various types oflenses under simulated, real-world conditions.

In clinical practice, many non-nominal conditions exist. These includecorneas with different aberrations, different amounts of IOL tilt anddecentration, and different pupil sizes for a nominal lightingcondition. Other conditions may apply in more unique circumstances.Randomly selected values of the above “conditions” were selected,individual MTFs calculated, and the average MTF tabulated. In effect,this procedure simulates the general clinical population and assessesthe complex interaction of the IOL surface design and aberrationsinduced by the non-nominal conditions.

FIG. 7 shows the results of such a “clinical simulation”, comparing theaspheric, spherical, and multi-zonal designs. FIG. 7 suggests that theaspheric design will improve the MTF at lower spatial frequenciescompared to the spherical design. From the patient's perspective,objects will have a higher contrast and color will appear richer. FIG. 7predicts that the multi-zonal design will provide even more improvementover a wide range of spatial frequencies. The patient should experienceboth improved contrast and visual acuity. The latter is related tochanges in MTF at about 100 cycles/mm. As expected, when averaged overan entire clinical population, the multi-zonal design provides moreimprovement compared to an aspheric design, even though the multi-zonaldesign is slightly lower in performance in the nominal condition (FIG. 6a).

In certain embodiments, a method of designing a multi-zonal monofocalIOL comprises providing an optical model of the human eye. The model mayinclude a corona, an iris, the IOL 60, a retina, and any liquids,substances, or additional devices between the these components. Themodel may also include various system design parameters such as thespacing between components and refractive index values.

The method further comprises providing an optical model of a lenscomprising an inner zone, an intermediate zone, an outer zone, and zonaldesign parameters (e.g., the IOL 60). The zonal design parameters foreach of the zones may include, but are not limited to, a radius ofcurvature, surface polynomial coefficients, inner radius, outer radius,refractive index, and DOE characteristics. In certain embodiments, themodel may include additional zones along with their correspondingparameters. One of the zonal design parameter may also include thenumber of zones in the lens. The model may comprise the zones and zonaldesign parameters for an anterior surface of the lens, the posteriorsurface of the lens, or both surfaces of the lens.

The method further comprises adjusting the zonal design parameters basedon an image output parameter for one or more non-optimal states of thelens. Examples of non-optimal states include, but are not limited to,IOL decentration and tilt, and different corneal aberrations (e.g.,different corneal asphericities). Examples of image output parameterinclude, but are not limited to, the Modulation Transfer Function, spotradius, and/or wavefront error. Alternatively, a plurality of outputparameters may be used for evaluation while adjusting the zonal designparameters.

With the IOL in a non-optimal state, zonal design parameters such as thenumber of zones and zone radii may be adjusted to correct any aberrantlight rays entering the system entrance pupil. For example, in the caseof IOL decentration and a three-zone lens, the first zone radius andsecond zone radius are chosen such that the second zone falls within theentrance pupil. The zonal design parameters for the zones exposed bylight entering the system entrance pupil may be adjusted to compensatefor the aberrations produced by the non-optimal state. Preferably, thezonal design parameters are adjusted until the image output parameterobtains an optimized or threshold value.

The method may also include adjusting the zonal design parameters and/orthe other system design parameters of the optical model based on theimage output parameter for an optimal state of the lens. Such an optimalstate would preferably represent a condition in which the IOL iscentered along the optic axis of the eye and normal thereto.

The method may be realized using optical design software that is resideson a computer or other processing device. The optical design softwaremay be used to numerically ray-traces various sets of light rays throughoptical model and that evaluates the image formed on the retina.Recognizing that the modeled cornea has finite aberrations, the designparameters of the multi-zonal monofocal IOL may be adjusted to improvethe quality of the image formed on the retina in terms of the imageoutput parameter or in terms of a plurality of image output parameters.

The resulting lens from this design may produce slightly lower retinalimage quality when placed in the optimal state as compared to theoptimal design in the optimal state. However, such a non-optimal statedesign will still allow a lens to be produced that providessignificantly better performance than that possible using sphericaloptics. Thus, the non-optimal state design provides superior performanceover a greater range of non-optimal conditions as compared to theinitial optimal-design.

In certain embodiments, additional non-optimal states are used tofurther adjust the design parameters in order to provide a design thatis suitable of a particular condition or set of conditions. The resultsusing various non-optimal states may be used to provide a lens suitedfor a plurality of anticipated non-optimal states of an IOL within aneye or certain population of eyes having certain aberrations. Forinstance, the method may be used for testing the lens over a pluralityof corneal surface variations and dispositions of optical elements inthe eye's optical system using tolerance analyzing techniques.Additionally, all or part of the method may be repeated one or moretimes to modify zonal parameters and achieve a better average opticalperformance. Known algorithms, such as assigning weighting functions tothe various non-optimal states, may be used to provide a lens withdesired characteristics.

While embodiments of the invention have been disclosed for an IOLsuitable providing enhanced performance under non-optimal conditions,such as when the IOL is decentered from the optical axis of the eye,those skilled in the art will appreciate that embodiments of theinvention are suitable for other ocular devices such as contact lensesand corneal implants. For instance, the method of designing amulti-zonal monofocal IOL may be adapted for improving the performanceof contact lenses, which are known to move to different positions duringuse relative to the optical axis of the eye.

While this invention has been described with respect to various specificexamples and embodiments, it is to be understood that these are merelyexemplary and that the invention is not limited thereto and that it canbe variously practiced within the scope of the following claims.

1. A multi-zonal monofocal ophthalmic lens comprising: an opticcomprising a plurality of zones, including: an inner zone having a firstoptical power; an intermediate zone surrounding the inner zone andhaving a second optical power that is different from the first power bya magnitude that is less than about 0.75 Diopter; and an outer zonesurrounding the intermediate zone having a third optical power differentfrom the second optical power; the plurality of zones all disposed suchthat light entering the entire optic from a distant point source isfocused to substantially a single point; wherein the third optical poweris equal to the first optical power.
 2. A method of designing amulti-zonal monofocal ophthalmic lens, comprising: providing an opticalmodel of the human eye; providing an optical model of a lens comprisingan inner zone, an intermediate zone, an outer zone, and zonal designparameters, the inner zone, the intermediate zone, and the outer zonedisposed such that all light entering the inner zone, the intermediatezone, and the outer zone of the monofocal ophthalmic lens from a distantpoint source is focused to substantially a single point; adjusting thezonal design parameters based on an image output parameter for one ormore non-optimal states of the lens; and testing the intraocular lensover a plurality of corneal surface variations and dispositions ofoptical elements in the eye's optical system using tolerance analyzingtechniques.
 3. A method of designing a multi-zonal monofocal ophthalmiclens, comprising: providing an optical model of the human eye; providingan optical model of a lens comprising an inner zone, an intermediatezone, an outer zone, and zonal design parameters, the inner zone, theintermediate zone, and the outer zone disposed such that all lightentering the inner zone, the intermediate zone, and the outer zone ofthe monofocal ophthalmic lens from a distant point source is focused tosubstantially a single point; adjusting the zonal design parametersbased on an image output parameter for one or more non-optimal states ofthe lens; and repeating at least a portion of the method to modify zonalparameters and achieve a better average optical performance.